1. Field of the Invention
This invention relates to a refractory composition, particularly, to a composition suitable for forming, for example, a sliding gate serving to adjust the discharge amount of a molten metal in the casting of a molten metal.
2. Description of the Prior Art
In the continuous casting of a molten metal, a sliding gate for adjusting the discharge amount of the molten metal is mounted in the ladle or in the nozzle at the bottom of a tundish.
The structure of the sliding gate is described in, for example, Japanese Published Patent Application No. 60-99460.
Various refractory materials such as high alumina materials and alumina-carbon materials are used as the materials of the sliding gate. However, since the peeling resistance, which is a special example of the spalling resistance, is an important factor of the sliding gate, the high alumina materials or alumina-carbon materials fail to provide a satisfactory material of the sliding gate. On the other hand, magnesia-based refractory materials, which exhibit a high corrosion resistance with respect to molten metal or basic slag, are widely used as materials of various steel-making furnace members such as a converter. However, a magnesia-based refractory material is low in its spalling resistance and, thus, is almost never used as the refractory material for forming the sliding gate.
Attempts to improve the spalling resistance of the magnesia-based refractory materials are reported in, for example, Japanese Patent Publication No. 53-13643, Japanese Published Patent Application No. 55-11669, Japanese Published Patent Application No. 55-107749 and Japanese Published Patent Application No. 58-26073.
In Japanese Patent Publication No. 53-13643 mentioned above, it is proposed to use a magnesia-based refractory material in combination with a periclase spinel material. In this case, the spalling resistance may be improved to some extent. But, the mixed material mentioned fails to produce a satisfactory result when it is used as the material of the sliding gate which is used under conditions much more severe than those under which the general steel-making furnace members are used. In Japanese Published Patent Application No. 55-11669, a carbon material is added to a magnesia-based refractory material so as to improve the spalling resistance of the material. The spalling resistance may certainly be improved by the addition of a carbon material. In this case, however, the resultant refractory material fails to exhibit a satisfactory mechanical strength. In Japanese Published Patent Application Nos. 55-107749 and 58-26073, it is proposed to add Si, SiC--Al, Fe--Si, etc., together with a carbon material to a magnesia-based refractory material. In these cases, the spalling resistance of the refractory material is improved by the addition of a carbon material. Also, reduction in the mechanical strength of the refractory material caused by the carbon material addition is overcome by firing the mixture containing Si, etc., under a non-oxidizing atmosphere. However, the firing mentioned is costly. Also, the carbon material is oxidized during use of the sliding gate, leading to deterioration of the texture of the refractory material. Further, if the refractory material is used for the casting of steels which are highly corrosive to the refractory material, such as a Ca-added steel and various free cutting steels, it is impossible to obtain satisfactory results, just as in the case of using conventional high alumina material and alumina-carbon material.